1. Field of the Invention
The present invention relates to a signal processing method and an image capturing device for a signal output from an image inputting device which may typically be a CCD.
2. Background Art
During processing of a signal output from an image inputting device such as a CCD, a conventional signal processing method directly uses signals from photoelectric conversion elements called pixels which are formed by photodiodes or the like or uses signals obtained by converting these signals from analog to digital. The conventional signal processing method will be described with reference to FIGS. 13 through 17. FIGS. 13 through 17 show how light signals which are target image signals are transferred in turn until they are output to a display or storage device after input.
FIG. 13 is a conceptual drawing regarding the internal structure of an image capturing device, which is typically applied to a camera or the like, formed on a semiconductor substrate, showing an image inputting device 1 such as a CCD, an image processing device 2 which executes analog/digital conversion or the like for processing image signals output from the image inputting device 1, and a display or storage device 3 for image data. Plural photodiodes 11 which are photoelectric conversion devices are disposed inside the image inputting device 1, and each one has a coordinate (x, y). For simplicity, FIG. 13 shows areas at x=1 to 5 and y=1 to 5. In addition, there are plurality of vertical-direction charge transfer areas 12 for transferring charges in the vertical direction (the vertical direction in FIG. 13), and plurality of horizontal-direction charge transfer areas 13 for transferring charges in the horizontal direction (the horizontal direction in FIG. 13). Shown in FIG. 13 is a state that light signals corresponding to an image are input and in each photodiode 11, a signal charge because of light is generated, which means that charges denoted at Q (1, 1) through Q (5, 5) are generated in the photodiodes 11 which are located at the respective coordinates.
FIG. 14 shows a state that the charges in the photodiodes 11 have moved to the vertical-direction charge transfer areas 12. FIG. 15 shows a state that the charges in the vertical-direction charge transfer areas 12 have been transferred in the vertical direction and the charges generated in the photodiodes 11 expressed by the coordinates Y=1 have moved to the horizontal-direction charge transfer areas 13. FIG. 16 shows a state that the charges which used to exist in the horizontal-direction charge transfer areas 13 in FIG. 15 have been transferred, the charges Q have been converted into signal intensities S in the image processing device 2 and these signals have been fed to the display or storage device 3. FIG. 17 shows a state that as a result of repetition of the operation shown in FIGS. 15 and 16, all of the information in the photodiodes 11 has moved to the display or storage device 3. The conventional image capturing device operates as described above.
However, in the conventional structure, when very small dirt 21 has adhered to the photodiodes section, e.g., at the pixel which is at the coordinate (2, 2) shown in FIG. 13, the charge at this position alone becomes extremely small as compared to the surrounding charges and therefore appears as a black spot on a screen as shown in FIG. 18(a). Meanwhile, in the event that a crystal defect 22 of the semiconductor substrate is located at the pixel at the coordinate (4, 4) shown in FIG. 13 in the photodiodes section, since a leak current develops even in the absence of light, and the charge at this position alone becomes extremely large as compared to the surrounding charges and therefore appears as a white spot on the screen as shown in FIG. 18(b). These become visible on the screen, which is not a sufficient capability as an image inputting device and causes deterioration of a manufacturing yield of image inputting devices. While a solution to this problem has been dirt reduction, crystal defect suppression and the like during manufacturing steps, this makes the manufacturing steps complex or necessitates use of expensive manufacturing equipment, and eventually serves as a restriction against a reduction of a cost of manufacturing the image inputting devices.
As a solution to the challenges regarding defects described above, Japanese Patent Application Laid-Open Gazette No. H9-46601 describes a method of determining whether there is a defect or not by comparing a pixel signal corresponding to each pixel with an average value of the pixel signals surrounding this pixel. However, since this method requires comparison with the average value, when a noise level of the peripheral pixels as a whole varies, a decision regarding whether there is a defect may become different depending upon the noise level. Further, there is a problem that in the case of an image in which signal intensities sharply change between adjacent pixels, any area having a comparable signal intensity to the signal intensity of a particular pixel which is a boundary where signal intensities change is erroneously decided as a defect because the signal intensity of the particular pixel is greatly different from an average value of plurality of pixels, and that signal intensity compensation is performed based on this decision and it becomes difficult to enhance a resolution such as a contrast.